The invention is in the field of methods and apparatus for the production of hydrogen by thermal decomposition of water.
Hydrogen has long been viewed as an ideal combustible energy source. The product of hydrogen combustion in air is essentially water, although under some conditions traces of oxides of nitrogen may also be produced. Hydrogen combustion produces no carbon dioxide, a major xe2x80x98greenhousexe2x80x99 gas which figures prominently in concerns about the deteriorating state of the environment, particularly in urban areas. Recent developments in fuel cell technology, and the high overall energy efficiency of fuel cells, have helped to produce conditions favorable to the more widespread adoption and use of hydrogen fuel.
Production of hydrogen is now mainly carried out by reforming hydrocarbons, such as methane, and to a lesser extent, by electrolysis of water. These processes typically involve the use of energy sources that create pollution. For example, hydrocarbon consumption at thermal power stations is part of the hydrogen production chain in some electrolytic processes. As a result, there is a long-felt need for efficient methods to transform water, either directly or indirectly, into hydrogen.
There are a number of thermal processes for producing hydrogen from water that may make use of solar energy. One such process is the direct decomposition of water in solar furnaces at very high temperatures, typically in the range of 2200-2500xc2x0 C. Many of these processes are not very efficient, yielding only 10-15% of the available hydrogen. A number of references disclose methods of obtaining hydrogen from a solar-powered thermal water dissociation reaction by selectively extracting either the hydrogen or oxygen through a porous material such as metallic nickel. (see U.S. Pat. Nos. 5,397,559; 4,233,127; 4,053,576; 5,306,411; as well as xe2x80x9cPossibilities of Separating Water Thermaolysis Products in Solar Furnacesxe2x80x9d, Shakhbazov et al. 1977, Gelioteknika, Vol. 13, No. 6, pp. 71-72, UDC 621.472). A potentially important limitation to the commercial implementation of such diffusion methods is that the rate of gas separation depends significantly on the area of diffusion surface available. Very large diffusion or membrane surface areas may be required for large scale production of hydrogen. It may therefore be necessary to heat large diffusion areas to very high temperatures to allow the separation of the desired molecule to occur prior to cooling of the gasses, in order to prevent recombination of hydrogen and oxygen. The heating of such large areas may pose significant commercialization problems due to radiant energy loses and attendant inefficiencies.
U.S. Pat. Nos. 4,030,890 and 4,071,608 to Diggs propose separating hydrogen from oxygen by centrifugal forces. Diggs discloses a chamber for separating hydrogen and oxygen which has an oxygen outlet and a hydrogen outlet. The oxygen outlet is circumferentially located in the peripheral walls of the chamber close to the bottom of the chamber, at the end of the chamber where water vapor is introduced. The hydrogen outlet in the Diggs device is axially located at the top end of the chamber, at the opposite end of the chamber from the end where water vapor is introduced. This arrangement of hydrogen and oxygen outlets in the Diggs device is predicated upon a particular spacial distribution of oxygen and hydrogen gases in the vortex of the reaction chamber. However, the behavior of heated gases in a vortex tube is complex. In a process sometimes termed the Ranque effect, a gas stream may be separated in a vortex tube into two outlet streams, one of which is hotter and one which is colder than the temperature of the gas feed (see U.S. Pat. No. 1,952,281). In such a process, it is taught in the art that pressure and compositional gradients form in the tube both axially and radially with the result that the vortex core contains a cooled gas that flows in a direction opposite to the direction of flow of the heated gas at the periphery of the vortex. This effect may be used to separate vapors from a gas stream, as disclosed in U.S. Pat. Nos. 4,343,772 and 5,843,801. In contrast, U.S. Pat. No. 3,922,871 discloses alternative flow parameters that are suggested to produce the opposite stratification of gas temperatures within a vortex, with the hotter gas localized in the vortex core. Although this reference does not teach specific gas separations, it is illustrative of the variability that may be encounter in vortex gas flow.
There is a need for alternative methods and devices for production of hydrogen by thermal dissociation of water, particularly methods and devices that may be adapted for use with solar energy.
In one aspect, the invention provides a process for producing hydrogen from water including heating water to a water dissociating temperature to form a dissociated water reaction mixture comprising hydrogen gas and oxygen gas. A vortex is formed of the reaction mixture to subject the reaction mixture to a centrifugal force about a longitudinal axis of an interior space of a vortex tube reactor, so that there is radial stratification of the hydrogen gas and the oxygen gas in the interior space of the vortex tube reactor. Hydrogen gas is preferentially extracted from the reaction mixture at spaced apart points along the longitudinal axis of the interior space of the vortex tube reactor. Alternatively, the process may include preferentially extracting oxygen gas from peripheral portions of the vortex at longitudinally spaced apart points along the circumference of the vortex tube reactor. The water may be heated to a dissociating temperature with concentrated solar radiation focused on the vortex tube reactor, and the water dissociating temperature may be between about 1800xc2x0 C. and about 3000xc2x0 C. The reaction mixture may be contacted with a catalyst that catalyzes the dissociation of the water into hydrogen and oxygen. A vacuum may be applied to preferentially extract the hydrogen or oxygen gases.
In another aspect, the invention provides a vortex tube reactor comprising an elongate wall having first and second ends, the wall and ends together defining an interior space having a longitudinal axis and adapted to house a vortex. An inlet port is provided in the first end for tangentially introducing a gas into the interior space to initiate circumferential movement of the gas in the interior space about the longitudinal axis to form the vortex. A hydrogen draw tube may be provided concentrically located in the interior space along the longitudinal axis, the hydrogen draw tube being porous to hydrogen gas at longitudinally spaced apart points. Alternatively, an oxygen draw tube may be provided concentrically located in the interior space adjacent to the cylindrical wall, the oxygen draw tube being porous to oxygen gas at longitudinally spaced apart points. The reactor may be comprised of a refractory material adapted to withstand a water dissociating temperature.
Although solar energy may be used in accordance with the invention to disassociate water into hydrogen and oxygen, other sources of heat may be used. The radiant energy capture unit may also have application in solar distillation processes, and solar heated boiler systems used to generate steam for heating and for turbine use, especially in solar turbine electric systems.
By removing one of the dissociation products from within the vortex reactor, the equilibria of the dissociation reaction may be driven further to completion. The stratification in the vortex tube of the oxygen and hydrogen may also shift the equilibrium of the dissociation reaction towards completion. Preferably, operating temperatures in the final reactor are maintained at levels close to the primary reactor, to facilitate rapid equilibration. With inadequate residence time or fall off in temperature of the final reactor, residual material from the transition zone remaining in the final reactor can be separated at a heavy ends exit port and sent for recycle. With a well insulated system, the oxygen exiting the final reactor can be sent to one or more additional polishing stratification stages before being heat exchanged with feed water. The same can be done with hydrogen as needed.
Solar energy may be provided to the chemical process section by means of many banks of computer controlled mirrors, typically to produce temperatures of about 2500xc2x0 C. required to partially decompose water into hydrogen and oxygen.
Feed water for the process of the invention is advantageously purified by filtration by known means, preferably before passing to a first heat exchanger. Preheated water from the first heat exchanger may then be piped to a combination heat exchanger and solar distillation unit, where the water is further purified by distillation. The distilled water may then be cooled in the first heat exchanger, transferring thermal energy to the in-feed water. The water may then be further purified in a third stage, such as reverse osmosis. Purified water may then be sent to the radiant energy capture system which captures a major portion of the energy which would otherwise be lost from thermal radiation. The heated water from this recapture unit is then piped into a third specially designed heat exchanger which heats the water to super heated steam with heat from the final stage of the solar reactor. This steam continues through the first stage solar reactor under pressure of about 10 atmospheres. This piping leads to the orifice within the reactor which allows the pressure to build. This high pressure allows better thermal transfer and keeps the water from splitting due to its higher pressure.
Hydrogen can be removed from a point near the stabilization zone of the vortex by a smaller diameter inner draw pipe. This step may be assisted by the use of a comparative vacuum that is not so strong that it disrupts gas dynamics at intermediate and peripheral zones.
Internal tubing in the RH vortex tube may be used to effect gas separation In one embodiment, inner core diameter may be about 20-30% of vortex tube diameter. An effective single point of removal has been found to be about 25% of the distance down the core from the tangential inflow. The core may be porous or perforated and of extended length in order to promote removal of hydrogen from the vortex tube.
Reactions other than the dissociation of water may be accelerated or catalyzed in accordance with the present invention. For example, ammonia could be made to decompose in a similar manner. Methane reforming may be considered. Other systems are possible if the appropriate mass distribution applies to reactants and products.
Other heat sources can be used such as waste heat from thermal electric stations. Although this heat is lower than required for disassociation of water, it would provide a preheat to the process. Later heating stages could use electricity. Candidates for this include fossil fuel and nuclear power plants. Geothermal is another source of heat which would be non-polluting. Again geothermal could be used as a preheat to the process with secondary heating coming from geothermal electric, solar or other.
Accordingly, in one aspect, the invention provides a method for transferring concentrated solar energy or other source of thermal energy to a water vapor stream comprising a 1st stage reactor having a confined path, in which the vapor stream undergoes directional changes thereby forcing the vapor against the walls of the chamber, causing turbulence and effecting a more efficient energy transfer. This may be facilitated by a variety of reactor conformations, including helical, conical, or zig-zag paths, or by providing protrusions in the reactor that give rise to turbulence, or by other means in which directional changes force the vapor stream against the walls of the reactor under increased pressure, or in which increased turbulence increases the thermal energy transfer from the reactor walls. In some embodiments, enlarged surface areas may be provided within the vapor stream by means of fins protruding from the walls of the channels in which the vapor passes, to cause a more efficient energy transfer. An orifice may be provided, restricting the output of the 1st stage reactor, so that the vapor pressure is increased due to the orifice restriction, thereby increasing the density of the water vapor and increasing the efficiency of energy transfer to the water vapor. The increase vapor pressure may also help to prevent the disassociation of the water vapor, thereby increasing thermal conductance.
In an alternative aspect, the invention provides a method for disassociating water in a 2nd stage reactor, in which water vapor is partially disassociated and further thermal energy is absorbed. An orifice restriction may be provided on the input of the 2nd stage reactor, to restrict the vapor flow. A vacuum may be provided from the output of the 2nd stage reactor, to create a partial vacuum in the 2nd stage reactor, causing partial disassociation of the vapor stream. The partial vacuum may cause an endothermic expansion in the vapor stream, during which additional solar or thermal energy may be added to cause further disassociation. The path taken by the gas in the 2nd stage reactor may be adapted as described above for the 1st stage reactor to facilitate transfer of thermal energy to the gas. A catalyst may be provided within 2nd stage reactor to facilitate disassociation of the vapor stream.
In an alternative aspect, the invention provides methods whereby water vapor at dissociating temperatures, is subjected to centrifugal forces by high vortex swirl velocities in a 3rd stage vortex tube reactor (FIG. 25), such that there is stratification of the reaction mixture within the interior of the reactor, region 36, preferably leading to further decomposition of water vapor to hydrogen and oxygen by mass action. Hydrogen gas or oxygen gas, or both gasses, may be selectively removed from the stratification zones within the interior space of the vortex reactor, preferably along a portion of the axial length of the stratification zone, shifting the equilibrium of the dissociation reaction to further decompose the water vapor. In alternative embodiments, vacuum pumps (49 and 52 of FIG. 2) may be used to apply partial vacuums adjustably applied to the oxygen exhaust 39 (FIG. 2) and hydrogen exhaust 38 (FIG. 2) ports of the reactor 36, to facilitate decomposition of water vapor to hydrogen and oxygen. The pressure differential across the hydrogen collection tube may also be adjusted by varying the speed of the vortex (by adjusting parameters such as the number of injectors and the velocity of gas injection) and also by adjusting the vacuum on the tube, to optimize the collection of hydrogen. Concentrated solar or thermal energy may be continually fed to the 3rd stage reactor to cause further and continuous disassociation of water vapor. A catalyst may be provided within 2nd stage reactor to facilitate disassociation of the vapor stream, and the catalyst may be provided on a ceramic substrate. Automated controls may be provided to measures the amount of water vapor exiting the hydrogen and oxygen exhaust ports, and this information may be used to automatically adjusts the balance of partial vacuum between the hydrogen and oxygen exhaust ports of the reactor to minimize any detected imbalance, or otherwise optimize the values, to facilitate decomposition of water vapor to hydrogen and oxygen. In some embodiments, the vacuum on each of the hydrogen and oxygen exhaust lines of the 3rd stage reactor may be balanced, to maintain a controlled stratification zone. Similarly, the overall partial pressure in the 3rd stage reactor may be adjusted to maintain preferred disassociation and stratification conditions.
In various aspects, the invention provides adaptations to facilitate efficient energy transfer. In one aspect, the invention provides a shaped target 37 (FIG. 2) to facilitate thermal energy absorption from concentrated sunlight. The shape of the target may be concave, preferably being adapted to result in the capture of additional thermal and light energy. In another aspect, radiant energy losses may be ameliorated using a radiant energy recapture unit in which a light valve (16 and 20 of FIG. 4) allows concentrated solar energy to enter the reactor region through a window (FIG. 5), shaped prisms, or shaped reflectors d58 (FIG. 7A and FIG. 7B). The omnidirectional scattering of reflected light and radiated energy from the reactor may be redirected into the reactor region by optimizing total internal refraction, and angular reflection, from the window, shaped prisms, or shaped reflectors. Energy absorbed by the light valve (a window, shaped prisms, shaped reflectors or an equivalent thereof) may be used to preheat the inflow water to the reactor. These mechanisms may be kept cool with water passing through channels (16 of FIG. 4 and 63 of FIGS. 5 and 5A), and the heated water may be used as to preheat the reactor input. The light valve may be limited in size, to allow only the concentrated solar energy to enter, to the extent possible, in order to minimize outward energy losses. The area surrounding the reactor, other than the light valve, may be adapted to reflect radiant energy back to the reactor (18, 22, and 23 of FIG. 4). The energy reflector (16 and 17 of FIG. 4) may be cooled by a water stream, and the energy absorbed by the water stream may be used to preheat the water feeding the reactor. The outside of the reflector assembly (21 of FIG. 4) around the reactor, excluding the light valve, may be made of a reflective material, and any energy absorbed may be used to preheat the water feeding the reactor. The cooling water pressure within the flow channels (16 of FIG. 4) in the radiant energy recapture unit may be regulated to compensate for the variable flow of water through the system. The reactor may be adapted so that concentrated solar energy striking the support struts of the energy recapture unit may be reflected onto the reactor. Hot exhaust gases of hydrogen and oxygen from the reactor may be used in a high temperature heat exchanger, with the infeed water in the opposing flow direction. The gas stream in the heat exchanger may be adapted, as discussed above, to force the gases against the walls of the exchanger to effect more efficient energy transfer. The heat exchanger may be constructed of ceramic materials in which structural integrity is maintained by having a continuous structure which is self supporting in which long thermally conductive heat exchange pathways are layered upon each other with thermally insulating materials isolating the thermally conductive pathways therefore providing a structural integrity. Thermal breaks may be established, using thermally conductive material, along the length of the thermally conductive pathways of the high temperature heat exchanger, to provide thermal pathways though the insulating material (FIG. 11). A combination heat exchanger/distiller (8 of FIG. 2) may be used with lower temperature exhaust gases from the high temperature heat exchanger, in which the hot outflow gases are heat exchanged with the inflow water to cause the inflow water to be distilled. Concentrated solar energy focused on the heat exchanger/distiller may be used to heat the inflow water when insufficient energy is not available from the hot outflow exhaust gases. The low temperature heat exchanger may be made of metals such as stainless steel. Energy may be retrieved from the condensation of the steam from the combination heat exchanger/distiller, for use in heating (6 of FIG. 2) the infeed water. The steam may be cooled to close to the ambient infeed water temperature and further filtered by a reverse osmosis system and used to cool other processes such as the energy recapture unit. The infeed water may be preheated, for example by a counter flow method, to condense the hot steam from the heat exchanger/distiller and to preheat the water which feeds into the heat exchanger/distiller. The heat exchanger may form an integral part of the combination heat exchanger/distiller and may be used to recover the heat of fusion from converting water into steam. Water vapor may be condensing out of the hydrogen and oxygen air streams after disassociation, separation and cooling (50 and 53 of FIG. 2). The air steams may be cooled to condense water vapor. and the condensed water may be fed back into the process to help minimize the amount of water purification required in the overall system (by reducing the amount of new infeed water required). One aspect of the invention provides a method of reclaiming energy from a reflective surface, in which water is used to cool the surface and this water is in turn used to heat a reactor input reactor.